The genus Paracladopus was established based on the type species P. chiangmaiensis in 2006. The two Paracladopus species are distributed in Thailand and Laos; however, neither of them has been documented in China to date. During our field work in 2020, we collected a river-weed in Wuzhi Mountain, Hainan Province of China. After checking the morphological characters, it was identified as P. chiangmaiensis. Then, we assembled and annotated its chloroplast genome based on the genome skimming data. The results showed that the complete chloroplast genome was 133,748 bp with 35% GC content, consisting of 76 protein-coding genes, 30 tRNA genes, and 4 rRNA genes. A maximum-likelihood tree constructed based on the matk genes showed that WuMS109 was clustered with P. chiangmaiensis (AB537420, AB698348) without base difference and together with the remains of Paracladopus formed a sister clade to Cladopus. This is the first report of P. chiangmaiensis that represents a new generic record for China. The discovery of this river-weed could lay the foundation for investigating their biogeographical patterns and species evolution in further studies.

aquatic, chloroplast genome, matK, new generic record, Paracladopus, river-weeds, Wuzhi Mountain

Podostemaceae, also known as river-weeds or podostems (Gustafsson et al. 2002; Koi and Kato 2012), which are the largest family of strictly aquatic angiosperms (De Mello et al. 2011), grow attached to waterworn rock surfaces, stones or even wood in rapids and waterfalls with pristine hydrology and high water quality (Rutishauser 1995; Koi and Kato 2020; Xue et al. 2020). They are found worldwide in rivers or streams with open and sunny habitats (Werukamkul et al. 2016), but distributed mainly in the tropical to warm-temperate regions with seasonal climates (Kato 2011). The species diversity center of river-weeds is located in the equatorial region of South America (De Mello et al. 2011). Its habitat and morphology have obviously undergone high specializations and extensive reductions (Gustafsson et al. 2002). These species lack double fertilization and the vascular tissue is reduced or lost (Johri et al. 1992; Kato 2016; Khanduri et al. 2016). The vegetative body of most river-weeds resembles algae, lichens, or mosses. So, they are easily overlooked and more field works are needed to investigate the species diversity. River-weeds are usually not obvious at conventional demarcation of stem, leaf and root (Willis 1902). The interpretation of the vegetative body also evokes much controversy (Mohan Ram and Sehgal 1992; Ota et al. 2001; Sehgal et al. 2002). In this text, we adopt the classical root-shoot model with its structural categories ‘roots’, ‘shoots’. The term ‘root’ refers to photosynthetic structures when endogenous shoot buds are developed but there are no exogenous leaves. Shoots are apparently absent or reduced, borne adventitiously on root. The Podostemaceae show an amazing diversity of root types. They vary from thread-like to ribbon-like and further to crustose (Mathew and Satheesh 1997; Jäger-Zürn 2000; Koi et al. 2019; Krishnan et al. 2019).

The life cycle of river-weeds is dictated by high and low water periods (Lalruatsanga 2020). During the rainy season, the plants submerge in violent currents exclusively in the vegetative phase (Kato 2016). When the water level lowers during the dry season, the plants emerge from the water and enter into the reproductive phase (Kelly et al. 2010; Koi and Kato 2012, 2015). In the early dry season, the plants form flower buds underwater, and subsequently come into flower and fruit when exposed to the air. The exposed plants wither and die while seeds are dispersed from their capsules. They are annual herbs, but perennial when submerged all year round (Kato et al. 2019). Exposure is necessary for reproduction, otherwise the plants will be unable to complete their life cycle (Kato 2016).

Podostemaceae comprises ca. 351 species (excluding infraspecies) assigned to ca. 50 genera in three subfamilies (POWO 2021). Podostemoideae, the largest subfamily (ca. 43 genera and ca. 322 species) of Podostemaceae, is distributed worldwide; subfamily Tristichoideae (6 genera and ca. 28 species) is mainly distributed in Asia, with the exception of Tristicha trifaria (Bory ex Willd.) Spreng., which is widely dispersed from Central and South America to Africa; while subfamily Weddellinoideae (a monotypic genus) is confined to South America (Kato 2006a). The phylogenetic position of Podostemaceae has been revealed by molecular data. They were included in the order Malpighiales, forming a sister group with Hypericaceae and related to Clusiaceae and Callophyllaceae based on plastid and nuclear markers or plastoms (APG IV 2016; Magallón et al. 2015; Li et al. 2021). However, Podostemaceae was a more basal isolated-clade in rosids and was recovered as Podestemales according to nuclear genes (Baker et al. 2021) or genomes (Xue et al. 2020).

The genus Paracladopus M.Kato, belonging to subfamily Podostemoideae, was established by Kato in 2006 based on the type species P. chiangmaiensis M.Kato, which was first discovered in northern Thailand (Kato 2006b). Two years later, another new species P. chantaburiensis Koi & M.Kato was discovered in southeastern Thailand (Koi et al. 2008). The two Paracladopus species are distributed in Thailand and Laos; however, neither of them has been documented in China to date. The ribbon-like roots and ellipsoidal or globose capsule of Paracladopus are similar to Cladopus H.Möller, but can be distinguished by its capsules with ribs or stripes, shoots borne on the sinuses of root branching and flanks of the root between successive root branches, and holdfasts presenting on ventral surface of root under the shoots (Koi and Kato 2012, 2015). Molecular phylogenetic analysis showed that Paracladopus was sister to Cladopus, and the two genera were in turn sister to the ‘Hydrobryum’ clade (including Ctenobryum Koi & M.Kato, Hydrodiscus Koi & M.Kato, Hanseniella C.Cusset, Hydrobryum Endl. and Thawatchaia M.Kato, Koi & Y.Kita) (Koi et al. 2012, 2019).

During a field trip to Wuzhi Mountain, Wuzhishan City, Hainan Province, China in December 2020, a river-weed (Fig. 1A-C) without flower and fruit attached to rock surfaces in rapids caught the first author’s attention. At first sight, it resembled Cladopus species by its ribbon-like structure; however, ensiform leaves and shoots borne on both sides of the ribbon-like structure (Fig. 2A, B) confused us. In March 2021, we revisited Wuzhi Mountain and found a few residual flowers and a large number of mature fruits. The flower possessed a single stamen and a single pistil with 2 stigmas (Fig. 2D, E). The fruit was globose with some stripes. After a survey of literature, we confirmed the morphological characters of the plants matched that of the species Paracladopus chiangmaiensis, distributed in Thailand and Laos. This species and genus has not been previously reported to occur in China. So, its discovery in Hainan Province represented the first record of this genus for China.

Figure 1. 

The natural habitat of Paracladopus chiangmaiensis in Wuzhi Mountain A, B the river-weed submerging in water with open and sunny habitat C the river-weed attached to waterworn rock surfaces in rapids D several individuals emerging from water and enter into reproductive phase.

Figure 2. 

Paracladopus chiangmaiensis in natural habit A inconspicuous shoots are observed B ribbon-like roots with tufts of leaves on flanks C flowering shoots on flanks of root D, E flowers with indehiscent anthers F nearly ripe fruits with dehiscent capsule valves.

We would like to thank Prof. Qinshi Zhao and his research team members Shuai Jiang, Liyan Peng for organizing a botanical expedition to Wuzhi Mountain. Mingsong Wu is indebted to Liu He for her continued support. This work was financially supported by CAMS Innovation Fund for Medical Sciences (CIFMS) [Grant No. 2021-I2M-1-032], The 2017 TCM Public Health Service Subsidy Special ‘National TCM Resources Survey Project’ [Finance Agency (2017) No. 66].